Animal fluency networks and item-based trajectories in people with Parkinson's disease before and after deep brain stimulation surgery

Consistent decline in animal fluency occurs in people with Parkinson's disease (PD) post-surgery for Deep Brain Stimulation of the subthalamic nucleus (STN-DBS). Recent findings indicate that patients produce fewer words after (vs. before) surgery. Also, they produce smaller clusters of related animals, fewer switches between clusters, and more frequent (lexical frequency) and earlier-acquired words. In this paper, we further investigated difficulties in lexical retrieval by reporting on word networks (to study the structure of the mental lexicon) and item-based trajectories (to understand how the linguistic characteristics of the task change as the task progresses) of 61 Dutch-speaking individuals diagnosed with PD before and post-surgery for STN-DBS and 40 age and education-matched healthy controls. Word networks contained a higher number of edges and nodes for healthy controls relative to people with PD post-surgery (adjusting for differences in sample size). Also, patients before surgery produced a network with a lower cluster coefficient relative to healthy controls, as well as to people with PD post-surgery. Item-based trajectories showed that both healthy controls and people with PD produced words of increasing complexity as they proceed with the task (e.g., less frequent, acquired later, of higher length). Also, healthy controls showed a steeper increase in the age of acquisition of words across the first ten responses compared to PD post-surgery. The re-emergence of structure (clustering) in word networks post-surgery for STN-DBS and the differences in item-based trajectories (vs. healthy controls) represent novel markers of lexical retrieval that are affected by surgery for STN-DBS and PD.